MRI systems, used for diagnostic imaging, usually use superconducting coils to generate the necessary high intensity DC magnetic field and some type of a shield to prevent stray field from entering spaces where the general public has access. The maximum field to which the public may be exposed is 0.5 MT (5 Gauss) and the magnet shielding system must control the stray field under all operating conditions.
There are two basic methods for shielding the MRI magnet. The first method is called passive shielding and involves placing a conventional ferromagnetic shielding (usually iron) around the MRI superconducting magnet in form of a return Yoke.
The second method of shielding magnets is called active shielding. In active shielding MRI systems there are two sets of coils: a first set of coils is responsible for the main homogenous magnetic field and a second coil system is used as the active shielding.
Usually the coils are made of low temperature superconductive material. If such a magnet would be subdivided by resistors, then in the event of a quench, the different currents in the different magnet sections would cause the 5 Gauss contour temporarily to appear much further from the magnet than in the normal running condition. One solution of those skilled in the art required building active shielding magnets from excessively heavy and expensive conductors. Thus there remains a need for means for protecting the superconducting coils in active shielding MRI systems from burning up during a quench without using heavy and expensive conductors and without increasing the stray field created by such systems.
An inherent problem associated with superconducting coils is protecting them against burn-out in the event of a quench. A quench is superconductor is caused by a mechanical disturbance in the superconductor itself, which causes a loss of superconductivity. This phenomenon results in a process where energy stored in the magnetic field is dissipated as heat in the coils. If the coils are not properly protected there is a risk that they will burn out during a quench.
For preventing the coils from burning out during a quench in passive systems two solutions are suggested. The first solution is the use of excess copper stabilizer in the superconducting wires. This protective method is expensive. The second solution uses internal subdivision of the superconducting coils and shunting by means of protective resistors and diodes. For example some prior art systems use semi-conductor diodes for this purpose and other prior art systems use ohmic protective resistors for the same purpose. These arrangements do prevent burn out in passive systems but fail to control the stray fields and do not address the quench problem of active systems.